Retardation elements have the ability to change linearly polarized light oscillating in a certain direction to a different direction and have the ability to convert circularly polarized light or elliptically polarized light to linearly polarized light. These abilities can be used, for example, to improve the viewing angle and contrast of liquid-crystal displays (LCDs). Specific examples known for the utilization of the ability of retardation elements to change polarized light include the use of a retardation element known as a ½-wave plate as a polarization rotator of a polarization beam splitter in a liquid-crystal projector, as disclosed in Patent Document 1, and the use of a retardation element known as a ¼-wave plate to convert the circularly polarized light obtained from a cholesteric liquid crystal to linearly polarized light, as disclosed in Patent Document 2. It is known that this ¼-wave plate can also be used in combination with a polarization plate as a circular polarization plate, in, for example, an anti-reflective filter and so forth.
This retardation element can be exemplified by retardation elements provided by subjecting the heretofore known plastics, e.g., polycarbonate, polyarylate, polyether sulfone, cycloolefin polymer, and so forth, to a monoaxial or biaxial stretch. These are generally called retardation plates or retardation films.
The characteristics of a retardation element can be expressed, for example, by the retardation value, which is determined by the product of the thickness of the element and the birefringence wherein the birefringence is the difference between the refractive index in the slow axis direction (in-plane direction in which the refractive index is the largest) and the fast axis direction (in-plane direction orthogonal to the slow axis direction). Retardation elements have recently been fabricated by causing the alignment of a liquid-crystal compound in a prescribed direction and immobilizing the alignment regime, as disclosed in Patent Documents 3 to 7.
Retardation elements that use liquid-crystal compounds can be made into thin films that cannot be achieved with plastic films. They have also been receiving attention because they characteristically enable the realization of complex alignment regimes that cannot be realized by the stretching carried out with plastic films.
It is known that the viewing angle characteristics, color, and contrast of various types of liquid-crystal displays can be improved by the use of retardation elements that employ such liquid-crystal compounds. For example, Patent Document 3 discloses an improvement in the viewing angle characteristics of a twisted nematic (TN) type liquid-crystal display that uses a retardation element that has a hybrid-aligned discotic liquid-crystal layer. Color compensation in a super twisted nematic (STN) type liquid-crystal display is disclosed in Patent Document 4. Patent Document 5 discloses an improvement to the viewing angle of an electrically controlled birefringence (ECB) type liquid-crystal display that uses a retardation element that has a hybrid-aligned liquid-crystal polyester. Patent Document 6 discloses an improvement to the viewing angle characteristics of optically compensated bend (OCB) type liquid-crystal displays and vertically alignment (VA) type liquid-crystal displays that use twisted-alignment liquid-crystal layers that exhibit a selective reflection wavelength region in the ultraviolet region. The disclosure in Patent Document 7 relates to a compound that is used in liquid-crystal compound-based retardation elements that are employed for compensation in the aforementioned liquid-crystal displays, and also to a method of producing this compound.
It is known that the use of a liquid-crystal polymer-based retardation element in automotive windshield glass can improve upon the problem of the reflected image presenting as a double image due to the two surfaces of the windshield glass, i.e., the inside and outside surfaces. For example, an automotive windshield glass is disclosed in Patent Document 8 that employs an optically functional film comprising a retardation element in film form laminated between two interlayer films.
In the case of these liquid-crystal displays, automotive windshield glass, and so forth, the previously mentioned retardation value is critical for enabling the retardation element to perform the appropriate polarization conversion at the target wavelengths.
However, a problem encountered with this retardation value is that it is changed by various ambient conditions, e.g., a high-temperature atmosphere, a high-temperature, high-humidity atmosphere, and so forth. Taking, for example, the case of the liquid-crystal displays used for automotive instrument panels and liquid-crystal projectors, the change in the retardation value due to exposure to a high-temperature atmosphere causes problems such as a decline in the contrast and viewing angle characteristics of the liquid-crystal display, and there is strong desire for a solution to this problem.